Treatment of Pulp

ABSTRACT

A method of treating a fibre pulp mixture comprises: (a) separating the fibre pulp mixture into at least two fractions to form a coarse fraction and a fine fraction; (b) bleaching the coarse fraction from (a); and (c) precipitating an alkaline earth metal carbonate in the fine fraction from (a). The products from steps (b) and (c) may combined in an integrated paper making process.

FIELD OF THE INVENTION

The present invention relates to methods for treating pulp for use in the production of paper and paper products, in particular super calendered (SC) paper. The present invention also relates to products obtained by said methods.

The term “paper” should be understood to mean all forms of paper, including board, card, paperboard, and the like.

BACKGROUND

One of the most important characteristics of paper is its degree of whiteness. Generally, and depending on the application, the whiter the paper, the higher the selling price. In order to obtain whiter papers, efforts have been directed to treatment of pulp by subjecting it to bleaching agents such as alkaline hypochlorite.

There are numerous disadvantages associated with bleaching pulp in order to increase the whiteness of paper. For example, in bleaching pulp, large quantities of bleaching agent are required. This contributes to increased production cost, as well as being an inefficient use of large amounts of bleaching agents.

Due to the costs of using current bleaching techniques, a more economical bleaching technique is required. Ideally, the provision of more economical bleaching techniques should not result in the overall paper making process becoming significantly more expensive and should not be at the expense of the environment. In addition, any improvements with respect to the bleaching process should preferably not result in a significant deterioration in the performance of the paper. Balancing these aspects has hitherto proven difficult.

One of the other environmental issues relating to the production of paper concerns the aqueous waste streams produced by paper mills, which generally comprise suspensions of fine organic microfibres, for example cellulosic fibres which typically have a length of no greater than about 75 μm, and other organic materials usually in association with inorganic particulate materials. The solids in such streams have, in some cases, proven difficult to dewater; in addition the waste streams are environmentally and economically undesirable to discharge. Aqueous wastes from paper mills generally comprise solids which are so-called fines which may be organic and/or inorganic in nature and which are defined in TAPPI Standard No. T261 cm-90 “Fines fraction of paper stock by wet screening” (1990). This document describes a method for measuring the fines of paper making stock and specifies that fines are those particles which will pass through a round hole of diameter 76 μm. In this particular definition “particles” includes minute materials of all types including inorganic particles, organic microfibres and particles and fine minerals.

The present invention is based on the finding that more economical and environmentally friendly methods for bleaching pulp can be obtained when the pulp to be treated in the bleaching process is divided into fractions determined primarily by the size of the fibres or particles in the pulp. More particularly, the pulp is divided into a coarse and a fine fraction followed by separate treatment of each of the fractions.

By separating the pulp into fractions as a function of the size of the fibres the amount of bleaching material required is decreased. This improvement in known bleaching techniques is achieved without a significant compromise in the qualities of the resulting paper products and in some respects, the qualities of the paper products are improved.

Advantageously, the present invention is particularly designed to operate in a continuous fashion at an integrated paper mill. In such an operation, delays between the various steps can be kept to a minimum and the potential so-called alkali darkening of mechanical pulp can be avoided or at least lessened.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a method of treating a fibre pulp mixture comprising:

-   (a) separating the fibre pulp mixture into at least two fractions to     form a coarse fraction and a fine fraction; -   (b) bleaching the coarse fraction from (a); -   (c) precipitating an alkaline earth metal carbonate in the fine     fraction of (a).

Preferably, the coarse fraction and/or the fine fraction are dewatered prior to (b) and (c) respectively.

Separating the fibre pulp mixture into a coarse and fine fraction separates the fibres of the wood pulp in the fibre pulp mixture by any one of a number of parameters related to the size of the fibres and may be carried out using known and standard screening techniques. Separation may, for example, be according to the length of the fibre, so that fractions comprising mainly long and short fibres respectively are obtained in each fraction.

The fractionation may be achieved by separating the fractions into fibres and particles having an average size above and below a certain value, for example, about 50 μm according to the TAPPI Standard No. T261 cm-90 mentioned previously. Accordingly, the fine fraction may comprise about 15-50% by weight, preferably 20-30% by weight of the fibres or particles making up the fibre pulp mixture prior to fractionation and will have an average particle size of less than about 50 μm. Preferably, the fine fraction will consist predominantly of organic fines which will pass through a substantially round hole of diameter 50 μm.

In step (c) of the first aspect of the present invention, the precipitated alkaline earth metal carbonate will usually form an aggregate with the fibres and/or particles of the fine fraction.

In a further aspect of the present invention, the aggregated material resulting from step (c) may be supplied for use as a filler or pigment in a filler or pigment containing composition. For example, the wet precipitate of alkaline earth metal carbonate and entrained fine material is added to a paper making composition to provide filler particles for the paper making fibres. Alternatively, the wet material may be dewatered by any conventional method, for example by pressure filtration or in a centrifuge.

In a further aspect of the present invention some or all of the mixed aggregated material resulting from step (c) may be combined with the bleached coarse fraction resulting from step (b) according to the first aspect of the invention, in order to make paper. This aspect of the invention advantageously results in minimising the various waste streams and helps to maintain the balance of the plant in which the paper making process is being carried out.

There are numerous other advantages associated with the present invention. Although bleaching may be performed in the normal manner, the bleaching equipment may be smaller in size compared to conventional bleaching units, due to the smaller mass to be bleached. In addition, as the coarse fraction will generally dewater more efficiently than the original pulp, the devices for carrying out the dewatering may also be reduced in size. In general, the bleaching chemical consumption for a given weight of material is proportional to the surface area of the material. The removed fine fraction will in general have a higher specific surface area than the coarse fraction and consequently the bleaching chemical consumption for the coarse fraction is lower for a unit of weight than that for the original pulp; it is estimated by about 10-15%. The investment costs for a bleaching plant according to the process of the present invention may be significantly less than the costs for the current type of plant; typically up to about 30% less. It is estimated that the running costs, mainly due to the decreased use of resources, for example, chemicals and electricity would be about 15% lower than for a conventional plant.

DETAILED DESCRIPTION OF THE INVENTION The Fibre Pulp Mixture

There are three major types of pulping methods known in the pulp and paper industry. These are (i) chemical, (ii) mechanical and (iii) a combination of chemical and mechanical.

The present invention is concerned with treating mechanical pulp. It is well known to persons of ordinary skill in the art of making pulp and paper how to make a mechanical pulp. The first step in the mechanical pulping process is the grinding or refining of wood. Some examples of mechanical pulp processes are set out below.

The Stone GroundWood (SGW) process involves making pulp by pressing logs and chips against an abrasive rotating surface. Originally, the grinding surface used was an actual stone. In current practice, specifically designed “artificial” pulp stones are available for the grinding.

A Pressurized Ground Wood (PGW) process is where the grinding operation is completely pressurized.

Another type of mechanical pulping is Refiner Mechanical Pulp (RMP) featuring atmospheric refining with no pre-treatment of the wood chips.

Thermo Mechanical Pulping (TMP) is a mechanical pulping process that evolved from RMP and a high temperature process known as the Apslund process. Thermo Refiner Mechanical Pulping (TRMP) is a variation in Thermo Mechanical Pulping. In this case, the chips are preheated under pressure and refining is carried out at atmospheric pressure.

De-inked pulp (DIP) is also suitable for use in the present invention. Typically for a DIP, the paper is disintegrated into fibres in a mechanically operating pulping drum, followed by screening, flotation, washing and bleaching. The mineral content of the fines fraction in DIPs can be as high as about 50 wt %.

Separating the Fibre Pulp Mixture

According to the present invention, the pulp is divided or screened into coarse and fine fractions; this may involve using any of the known fractionating or screening methods used in pulp processing, for example a wire screen possessing a particular, or range, of hole size or a number of wire screens possessing a range of suitable hole sizes. Numerous methods are known and are suitable for separating fibres of varying size. For example, in US 2004/0011483, the contents of which are herein incorporated by reference, a method, apparatus and a screen for screening mechanical fibre pulp is described which are suitable for use in the present invention. Further examples include Dorr Oliver's DSM curved screen and Krofta's spray screen.

In the method described in US 2004/0011483, a mechanically manufactured fibre pulp mixture containing fibres of varying lengths is screened into at least two fractions containing fibres of mainly varying lengths. The invention described in US 2004/0011483 allows for the accurate separation of a fibre fraction that is shorter than a fraction representing a particular length from the fibre pulp mixture. In particular, the apparatus described in US 2004/0011483 comprises a gap screen to separate short fibres comprising a convergent gap and at least one wire restricting the convergent gap. The fibre pulp mixture to be screened is fed into the convergent gap so that it flows through the gap screen, in the same direction as the wire, towards the convergent end of the gap so that the shorter fibres and some water exit through the openings in the wire and the remaining part of the fibre pulp mixture exits from the gap screen through the output port at the convergent end of the gap.

Other methods and apparatus suitable for separating the pulp into fractions of differing size are well known to those skilled in the art and are suitable for use in the present invention.

There are numerous ways in which particles, and fibres may be measured. According to the present invention the screening of the fibre pulp mixture will, for example, result in the coarse fraction being 20-30% by (dry) weight less when compared to the original fibre pulp mixture. The screening method will be such that preferably the fine fraction will consist predominantly of fibres and/or particles that are about 50 μm or less; i.e. most of the 20-30 wt % loss will be substantially in the form of fines according to TAPPI Standard No. T261 cm-90.

Bleaching the Coarse Fraction

After separating the coarse and fine fractions, the coarse fraction is bleached in a standard manner. Prior to bleaching, the coarse fraction may be dewatered and optionally washed using any of the known standard procedures, for example, by use of a belt press. Dewatering of the coarse fraction may advantageously give rise to better economy and efficiency of the overall process. Depending, for example, on the type of solids present, dewatering of the coarse fraction typically results in the solids content in the coarse fraction increasing from a range of about 0.01 to 0.1 wt % to a range of about 1.5 to 6.5 wt %. Due to the reduced mass of the original pulp mixture, the equipment needed for the bleaching process may advantageously be smaller than standard bleaching equipment. Bleaching processes are well known to those skilled in the art and those processes suitable for use in the present invention will be readily evident. For example a reducing bleaching agent may be used such as a dithionite salt, for example sodium or zinc dithionite. Other examples of reducing bleaching agent include zinc dust, thiourea dioxide (i.e. formamidine sulphinic acid) and sulphur dioxide. Oxidising bleaching agents may also be used and hydrogen peroxide is a particularly preferred bleaching agent. Other suitable oxidising bleaching agents include peracetic acid and ozone. The amount of the reducing bleaching agent used is preferably in the range of 1.5 g to 7.5 g of the reducing bleaching agent per kilogram of dry fractionated pulp material and in the case of oxidising bleaching agents the corresponding amount is preferably in the range of 1.0 g to 4.0 g of the oxidising agent.

The Fine Fraction

Following separation of the pulp mixture, the fine fraction may be in the form of a dilute suspension. It is generally unacceptable, for environmental and economic reasons, to allow such a suspension of fine particles to be discharged to rivers or lakes, and as a result such unwanted suspensions of very fine particles are ultimately often retained in lagoons, thus occupying large areas of land which could more profitably be used for other purposes.

In the present invention, the fine fraction may be thickened to a suitable level, for example by a known dewatering method, for example by using Dissolved Air Flotation (DAF) Unit(s) and a mineral filler is co-precipitated on its surface using techniques known to those skilled in the art. For example, the applicant's rECClaim™ method is a known method and precipitates an alkaline earth metal carbonate (preferably calcium carbonate) on the surfaces of paper making fines in controlled conditions and is suitable for use in the present invention. These processes are disclosed in numerous patents and patent applications including U.S. Pat. No. 5,830,364, the entire contents of which are hereby incorporated by reference. Anions other than carbonate may also be used in the present invention. An example of such an anion is silicate.

Dewatering of the fine fraction may advantageously result in a better quality carbonation product and provides cleaned water for recycling in the process of the present invention. In the present invention, the solids content of the fine fraction is generally increased from about 0.03 wt % to 6.5 wt %, more particularly to a range of about 0.2 wt % to about 4 wt %, for example to about 1.5 wt %; in particular this increase may be achieved with the use of DAF units. The use of a combination of dewatering units, such as DAF units, may advantageously lead to the use of units of reduced size and/or the amount of waste solids in the overall process being reduced.

In the method described in U.S. Pat. No. 5,830,364, the so-called fines material is generally considered to become entrained in the alkaline earth metal carbonate to form a mixed aggregated material. It is known to further incorporate this mixed aggregated material in any one of a number of compositions; for example paper making compositions, paper coating compositions, paint compositions or plastics compositions.

The alkaline earth metal carbonate precipitate may be formed by introducing into the suspension of the fine fraction a source of alkaline earth metal ions and a source of carbonate ions. This will form the desired precipitate of alkaline earth metal carbonate in situ which will entrain the particulate or fibrous material. The first reagent which is added, should preferably be uniformly distributed throughout the aqueous based fine fraction to avoid local concentration gradients. When the first reagent is sparingly soluble, as is the case with calcium hydroxide, thorough mixing is desirable. It is also desirable that the fine fraction, which may be in the form of a suspension, should be agitated while the second reagent is added in order to ensure an even distribution of the precipitate.

It is preferred to add the source of alkaline earth metal ions and the source of carbonate ions sequentially. The source of alkaline earth metal ions may be added first followed by the source of carbonate ions or the source of carbonate ions may be added first followed by the source of alkaline earth metal ions. U.S. Pat. No. 5,665,205, the contents of which are hereby incorporated by reference, includes a description of a process wherein carbon dioxide is brought into contact with a fibre slurry prior to the introduction of a calcium hydroxide slurry.

The source of alkaline earth metal ions is conveniently the alkaline earth metal hydroxide (known as milk of lime, when the alkaline earth metal is calcium), but it may alternatively be a water-soluble alkaline earth metal salt, for example the chloride or nitrate. The alkaline earth metal hydroxide may be added to the aqueous suspension already prepared, or alternatively may be prepared in situ, for example by slaking an alkaline earth metal oxide (e.g. quicklime when an aqueous suspension of calcium hydroxide is desired) in the suspension.

The source of carbonate ions is conveniently carbon dioxide gas, which is introduced into the fine fraction and, depending on the order of addition, the source of the alkaline earth metal ions. The carbon dioxide gas may be substantially pure as supplied in gas cylinders or may be present in a mixture of gases such as flue gases. Alternatively, the source of carbonate ions may be an alkali metal or ammonium carbonate. Sodium carbonate is particularly preferred.

It is particularly preferred that the source of alkaline earth metal ions is introduced into the fine fraction either by slaking an alkaline earth metal oxide, for example calcium oxide or quicklime, in the fine fraction or by adding to the fine fraction a separately prepared suspension of an alkaline earth metal hydroxide.

Further Processing of the Treated Coarse and Fine Fractions

Preferably the mixed aggregated material from step (c) is combined with the bleached coarse fraction resulting from step (b) according to the first aspect of the invention, in order to make paper. Typically the combined product when used in a conventional paper-making process will be mixed with chemical pulp in a paper machine mixing chest in order to make paper stock. From there, the paper stock is pumped to a machine chest, via cleaning and de-airing systems and is then pumped to a headbox. Mineral fillers and paper-making chemicals may be added to the paper stock at appropriate points during this process.

The mixed aggregated material from step (c) and the coarse fraction from step (b) may be used to make paper in separate paper making processes.

There are numerous types of paper, coated or uncoated, which may be made according to the present invention, including paper suitable for books, magazines, newspapers and the like. The paper may be calendered or super calendered as appropriate; for example super calendered magazine paper for rotagravure and offset printing may be made according to the present methods. Paper suitable for light weight coating (LWC), medium weight coating (MWC) or machine finished pigmentisation (MFP) may also be made according to the present methods. When DIP is used in the fibre mixture, typical types of paper, in addition to those mentioned above, include so-called woodfree paper. Also, as will be clearly evident to those familiar with the paper making art, the aggregated material may be blended in various proportions with conventional filler materials, e.g. precipitated or ground calcium carbonate, kaolin and other clay minerals, metakaolin, talc, calcium sulphate, the ingredients and composition being selected according to the quality of the paper required to be produced. In general these materials are likely to be in a slurry form when they are mixed.

The paper maker will normally select the concentration of the aggregate material in aqueous suspension and the delivery rate of the suspension at the point of addition to the paper making composition.

In general, the paper making composition may typically comprise, in aqueous suspension and in addition to the optional aggregated material referred to in the methods of the present invention, cellulosic fibres and other conventional additives known in the art. A typical paper making composition would contain up to about 35% by weight of filler material of the total dry contents and may also contain a cationic or an anionic retention aid in an amount in the range from 0.1 to 2% by weight, based on the dry weight of the filler material. It may also contain a sizing agent which may be, for example, a long chain alkylketene dimer, a wax emulsion or a succinic acid derivative. The composition may also contain dye and/or an optical brightening agent.

The paper products obtained according to the present invention may be coated. A paper coating composition will include, in aqueous or non-aqueous suspension, an adhesive and, optionally, other filler materials. The formula of the paper coating composition will typically depend upon the purpose for which the coated paper is to be used.

Calendering is a well known process in which paper smoothness and gloss is improved and bulk is reduced by passing a coated paper sheet between calender nips or rollers one or more times. The methods according to the present invention are particularly suited for the production of super calendered paper.

Methods of coating paper and other sheet materials, and apparatus for performing the methods, are widely published and well known. Such known methods and apparatus may conveniently be used for preparing coated paper. For example, there is a review of such methods published in Pulp and Paper International, May 1994, page 18 et seq.

The coating is usually added by a coating head at a coating station. According to the quality desired, paper grades are uncoated, single coated, double coated and even triple coated. When providing more than one coat, the initial coat (precoat) may have a cheaper formulation and optionally less pigment in the coating composition. A coater that is applying a double coating, i.e. a coating on each side of the paper, will have two or four coating heads, depending on the number of sides coated by each head. Most coating heads coat only one side at a time, but some roll coaters (e.g., film press, gate roll, size press) coat both sides in one pass.

Embodiments of the present invention will now be described by way of example only, and without limitation, with reference to the accompanying drawings and illustrative examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a conventional method for bleaching a mechanical pulp.

FIG. 2 is a flow diagram showing an embodiment of the process according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1, which illustrates a conventional method for bleaching a mechanical pulp, the mechanical pulp, typically TMP is led through pipeline (50) as an aqueous solid suspension at about 0.6 wt % solid content to a dewatering device (1) such as a disk filter. The thickened pulp is led through pipeline (51) to an unbleached pulp tank (2) at about 8 wt % solid content in an aqueous suspension. The water from the dewatering device (1) is led via pipelines (52) and (53) to clear (3) and cloudy (4) water tanks respectively. Cloudy water is further led through pipeline (54) via a feed tank (5) to a micro flotation system (6) where the suspended solids are collected and dumped via pipeline (55). The cleared water is fed back from the microflotation tank (6) to the mechanical pulp water chest (7) via pipeline (56) for re-use. Any extra water required for the system is taken from paper machine press section (6 a) to the feed tank (5) through pipeline (56 b) and then to microflotation tank (6). This operation also serves to dump the solids in the press section water. Extra water may also be taken from a paper machine clear filtrate chest (6 b) to the clear water tank (3) via pipeline 56 a. The thickened pulp from the unbleached pulp tank (2) is fed through pipeline (57) to a thickener (8), for example a belt press, to be thickened, i.e. dewatered into bleaching solids at about 35 wt % and bleached in a reactor (9) by standard means, for example, a peroxide method, via pipeline (65). Water from a bleached pulp thickening water tank (3 b) is fed via pipeline (58) to dilute the bleached pulp, in this case, to about 8 wt % solids and the pulp is washed in another thickener (10) after passing through pipeline (59). The water from water tank (3 b) is also used as washing water for the dewatering device (1) via pipeline 58 a. Washing water for the thickeners (8) and (10) is taken from the cleaned process water system pipeline (60). Thickened, washed and bleached pulp is diluted to, for example, 8 wt % solids after passing through pipeline (72) in to mixer (11) using process water via pipeline (73). The diluted pulp is then led via pipeline (74) to bleached pulp storage chest (12) for further use. Water from thickener (10) is fed through pipeline (71) to water tank (3 b). The water from thickener (8) is fed to the water tank (3 a) via pipeline (70). Water from the water tank (3 a) is fed via pipeline (70 a) to cloudy water tank (4). Water chest (7) supplies water to the clear water tank (3) and to the pulping process via pipelines (74) and (75) respectively.

In FIG. 2, the mechanical pulp is fed to a screening/separating device (15), for example an OptiThick™ washer (commercially available from Metso Corporation and as described in US 2004/0022483) via pipeline (50) and fractionated into coarse and fine fractions. The coarse fraction is fed from the screening device (15) to a bleaching process (9), via (optionally) a thickener (8) at, for example 8% by weight solids, through pipelines (51), (57) and (65). Bleaching is performed in the conventional manner. The fine fraction leaves the screening device (15) as a dilute suspension at, for example 0.2% by weight solids via pipeline (80). The separation to a cloudy leg and clear leg (features (3) and (4) according to FIG. 1) is not necessary. The fine fraction is optionally thickened, for example, in a dissolved air flotation (DAF) unit (16). In general, for carbonation of the solids to be successful, the solids need to be increased, for example, to about 4% by weight. The solids from the DAF unit (16) are fed via a pipeline (81) to a unit (17) that will co-precipitate the alkaline earth metal carbonate with the fine fraction. The co-precipitate is formed, for example, by adding milk of lime to agitated fines slurry, and bubbling carbon dioxide containing gas through the mix until slightly acid conditions are reached. Pipeline (16 b) feeds process water to bleaching plant thickeners. The water from the air flotation unit (16) has a very low suspended solids content, typically below 50 ppm, and is particularly useful process water and is led to the mechanical pulp water chest (7) or pipeline 16 b via pipeline (16 a). Part of the water from the air flotation unit (16) is stored in pulp plant water chest (7) and supplies water for the screening/separating device (15) via pipeline (74), and clear water to the pulping process via pipeline (75). The other feeds to the water chest (7) come from the microflotation tank (6) and from the paper machine clear filtrate chest (6 b) via pipelines (56) and (56 a) respectively. By using the, for example, DAF unit at this stage, the required microflotation capacity for the process is reduced significantly, and smaller equipment may be used. The carbonator feed solids are typically about 4 wt % resulting in a fines/calcium carbonate co-precipitate of about 6 wt % solids. Pipeline (85) takes the co-precipitate to a storage tank (18). The co-precipitate may be combined with the bleached coarse fraction from storage tank (12) in a paper making process.

Though not shown in FIG. 2, the combined product resulting from storage tank (18) and storage tank (12) when used in a conventional paper-making process will, for example, be mixed with chemical pulp in a paper machine mixing chest in order to make paper stock. From there, the paper stock is pumped to a machine chest, via cleaning and de-airing systems and is then pumped to a headbox. Mineral fillers and paper-making chemicals may be added to the paper stock at appropriate points during this process.

EXAMPLES Test Methods Brightness

The ISO brightness of coated paper was measured by means of an Elrepho Datacolour 2000™ brightness meter fitted with a No. 8 filter (457 nm wavelength), according to ISO 2470: 1999 E.

Brightness Cakes

Brightness cakes or pads were made according to standard methods. More specifically, the pads are formed by taking about 5 g by dry weight of sample and diluting to about 1%. The sample is dewatered on a 15 cm diameter Buchner funnel to give pads of approximately 280 g basis weight. The pads are then pressed on a handsheet press for about 2 minutes, using three standard blotters per pad. The sequence on the press is: 3 blotters; filter paper; sample; metal plate. After pressing, the samples are dried between two fresh blotters on a drying drum with a surface temperature of 80° C. The filter paper is removed from the dry pad and the brightness is measured from each side wherein the top is the side away from the filter paper and filter is the side against the filter paper. Reflectance measurements were carried out on the brightness pads.

Light Scattering

The light scattering coefficient represents the proportion of light that is scattered from a surface. Light scattering coefficients were measured according to ISO 9416: 1998E.

Light Absorption

The light absorption coefficient is a measure of the portion of incoming light that is absorbed by a surface. It is inversely related to the surface brightness and directly to its opaqueness. Light absorption was measured according to ISO 9416: 1998 E.

Yellowness

The yellowness was measured according to the procedure described above for the brightness measurements. The yellowness is reported as the value obtained when the reflectance at 457 nm is subtracted from the reflectance at 571 nm.

Paper Strength

Paper strength was assessed using standard tensile and tear tests. Surface strength measurements and tensile stiffness measurements were also carried out. Tensile index measurements and tensile stiffness index measurements were carried out according to ISO 5270: 1924-2 on an L&W Alwetron instrument. The tear index was measured according to ISO 5270: 1998 E on an L&W Elmendorf instrument.

Freeness Tests

The freeness test, which measures drainability, is a traditional method for characterising pulp. Common test methods for characterising pulp are the Canadian Standard Freeness (CSF) test method and the Schopper-Riegler method.

CSF Test Method

The CSF measurements were carried out according to ISO 5267-2:2001 E.

The Bauer-McNett (B-McN) Classification

The Bauer-McNett classification is a well-known method for characterising both chemical and mechanical pulps, although it is generally considered more suitable for the latter. The Bauer-McNett apparatus has been shown to classify pulp primarily according to fibre length. The results are represented as the portion of fibres having different freeness values and were obtained by making measurements according to SCAN-M 6:69. For example, in Table 1 below, with regard to the feed SGW, the percentage of fibres by weight having a freeness value of 30 or greater is 8.4% whilst the percentage of fibres by weight having a freeness value ranging from 50 to 30 is 13.9%. The values 30, 50 and 200 are the size of the meshes on the screening device. For example, B-McN+30(%) in Table 1 indicates the fraction (% by weight) that remains on a 30 mesh wire and B-McN+50(%) indicates the fraction that remains on a 50 mesh wire but passes through the 30 mesh wire. The size of the holes in the mesh wires are as follows:

30 mesh wire—11901 μm; 50 mesh wire—297 μm; 200 mesh wire—74 μm; fines—<74 μm.

Whilst the present invention is suitable for operation in a continuous fashion at an integrated paper mill, the examples presented below are illustrative of the primary steps which may be combined in the overall process.

Measurements were carried out on SGW pulp obtained from the Kajaani paper mill, (UPM-Kymmene Corporation, Finland) suitable for use in the manufacture of super calendered magazine paper on Paper Machine Number 2.

Example A Pulp Splitting

Pulp splitting was carried out using the OptiThick™ device which is commercially available from Metso in Inkeroinen, Finland, under the following conditions:

Speed—600 m/min; Head Box Feed—40 litres/s; Head Box Feed Pressure—21 kPa;

Feed Temperature—60° C.;

Wire—Kraftex CD288 S-39, fabric 3052203.1.

The properties of the SGW feed and the coarse fraction obtained after pulp splitting are set out in Table 1 below.

TABLE 1 B- B- Brightness CSF McN + McN + B-McN + B-McN Sample (% ISO) (ml) 30 (%) 50 (%) 200 (%) fines (%) SGW feed 68.2 41 8.4 13.9 27.2 50.5 Coarse 62.7 159 19.2 27.0 32.8 21.0 fraction

Example B Bleaching of the Coarse Fraction

The Stone GroundWood (SGW) coarse fraction which was obtained according to Example A above, was pretreated with 0.1 wt % diethylenetriaminepentaacetate (DPTA, available from E.G. Merck) and thickened to 25 wt % solids using a vacuum funnel and centrifuge under standard conditions. A series of chemical doses of hydrogen peroxide was used to bleach 30 g samples in plastic bags. Bleaching was carried out at 70° C. (maintained in a water bath), using 25 wt % solids and a reaction time of 90 minutes. Following this, the samples were acidified to pH 5.5 with H₂SO₄.

Brightness cakes of each of the samples obtained were made and the results according to Table 2 were obtained which compare the original feed with the coarse fraction.

TABLE 2 Residual Example Peroxide NaOH Silicate DTPA pH pH peroxide B'ness B'ness Number (wt %) (wt %) (wt %) (wt %) start final (wt %) start final Feed SGW 1 1.5 1.35 1.2 0.1 10.9 8.8 0.21 68.2 73.9 2 2.0 1.6 1.5 0.1 10.9 9.0 0.64 68.2 77.9 3 3.0 2.1 2.1 0.1 10.8 9.4 0.93 68.2 80.1 4 4.0 2.6 2.7 0.1 10.9 9.8 1.26 68.2 82.1 Coarse Fraction 5 1.5 1.35 1.2 0.1 10.9 8.9 0.28 62.7 75.7 6 2.0 1.6 1.5 0.1 10.9 9.0 0.68 62.7 79.2 7 3.0 2.1 2.1 0.1 11.0 9.4 1.15 62.7 81.4 8 4.0 2.6 2.7 0.1 11.1 9.7 1.47 62.7 83.6

It can be seen from Table 2 that comparable brightness of the coarse fraction when compared with the original feed material is achieved with a significantly lower hydrogen peroxide dose than required for the original feed material.

Based on the results from Table 2, a suitable dosage of chemicals for bleaching pulps for the production of hand sheets was determined. The procedure was carried out using continuously mixed bleaching reactors. The results are presented in Table 3 below.

TABLE 3 Residual Example Peroxide NaOH Silicate DTPA pH pH peroxide B'ness B'ness Number (wt %) (wt %) (wt %) (wt %) start final (wt %) start final Feed SGW  9 1.8 1.5 1.38 0.1 10.3 8.6 0.43 68.2 76.4 10 1.8 1.5 1.38 0.1 10.3 8.6 0.46 68.2 76.9 Coarse Fraction 11 1.5 1.35 1.2 0.1 10.8 8.8 0.45 62.7 76.8 12 1.5 1.35 1.2 0.1 10.8 8.8 0.43 62.7 76.2 13 1.5 1.35 1.2 0.1 10.8 8.7 0.44 62.7 75.8

Example C Carbonation of the Fine Fraction

A mobile carbonation unit was used to study the effect of carbonation on the optical properties of de-inking plant micro flotate. The unit consisted of:

two feed stock tanks each of 750 litres capacity;

a slaked lime preparation module which comprised a 100 litre stirred vessel, a centrifugal slaked lime slurry transfer pump, a 300 mesh screen for the lime slurry and a 100 litre stirred lime storage tank;

a carbonation unit which comprised feed pumps for micro flotate feed and milk of lime, a transfer pump for the precipitated product, carbon dioxide vaporizer and flow control equipment all operating with respect to a 10 litre turbine mixed carbonator vessel, that could be heated via an inside hot water coil; and

a product storage tank.

The unit possessed all of the controls necessary for carrying out the required operations and a small laboratory for quality control work.

The micro flotate samples were taken two and a half hours after coarse and fine screen rejects had been stopped from entering the micro flotate.

Typical batch carbonation operating parameters were as follows:

Reactor volume (litres) 10 Feed pulp volume (litres) 8.5 Mass of feed solids (kg) 0.2 Feed solids (adjusted in feed stock tank) (wt %) 2.35 Projected product solids (wt %) 4 Added carbonate in product (%) 50 Weight of added carbonate (kg) 0.2 Milk of lime concentration (moles/litre) 1.33 Volume milk of lime added (litres) 1.5 Weight of Ca(OH)₂ added (kg) 0.148 Weight of CO₂ added (kg) 0.088 Gas application rate (moles CO₂/min/mole Ca(OH)₂) 0.07843 CO₂ concentration (% vol/vol) 100 Gas utilisation efficiency (%) 85 Reaction rate (g carbonate/litre/min) 1.33 Reaction time (min) 15 Over carbonation time (min) 2 Turn round time (min) 10.

Samples were taken from the feed and from the product of each batch. About 100 cm³ of slurry was filtered through a 5.5 cm Buchner funnel using Whatman #50 filter paper. The wet pads were dried slowly in an oven at 50° C. The dried pads were used for optical measurements, and ash and calcium carbonate analyses.

Table 4 illustrates the feed quality over one day of running the carbonation process in which six runs were carried out. The column headed Ash refers to the amount of incineration residue from each sample at 900° C. and was measured according to ISO 2144:1997 E.

TABLE 4 Brightness Run CaCO₃ Kaolin Fibre Ash (ISO) Yellowness No. (wt %) (wt %) (wt %) (wt %) Filter Top Filter Top 1 18.9 41.3 40.6 46.2 48.4 46.3 12.0 12.9 2 17.8 42.5 39.7 47.1 40.3 50.6 12.0 9.2 3 15.9 43.4 40.7 46.8 50.9 52.6 11.3 9.0 4 17.6 42.1 40.3 46.6 48.2 51.4 11.4 9.4 5 18.3 42.4 39.3 47.2 51.3 53.5 11.2 9.4 6 17.8 41.9 40.3 46.6 50.5 52.0 10.5 10.6

Table 5 shows the feed samples bulked to one daily sample for three different daily runs. In Table 5, the number in parentheses after the run number indicates the total number of runs making up the bulk number.

TABLE 5 Run Ash Brightness No. CaCO₃ Kaolin Fibre (wt (ISO) Yellowness (Bulk) (wt %) (wt %) (wt %) %) Filter Top Filter Top 1 (6) 18.1 41.3 40.6 46.2 48.4 46.3 12.0 12.9 2 (7) 14.3 43.9 41.8 46.3 46.7 48.4 12.3 10.7  3 (20) 21.1 41.6 37.2 48.2 44.0 46.2 11.2 10.1

Each of the samples from Table 5 was used as feed material for the co-precipitation with calcium carbonate according to the process conditions set out above. The properties of the resulting products are set out in Table 6.

TABLE 6 Run Ash Brightness No. CaCO₃ Kaolin Fibre (wt (ISO) Yellowness (Bulk) (wt %) (wt %) (wt %) %) Filter Top Filter Top 1 (6) 59.8 19.3 20.9 50.3 68.4 68.0 10.6 10.6 2 (7) 58.4 19.8 21.8 49.9 65.4 64.3 11.4 11.8  3 (20) 56.6 21.4 21.9 50.3 64.3 63.9 11.0 11.4

The effect of added calcium carbonate on the brightness is summarised in Table 7.

TABLE 7 CaCO₃ B'ness Run No. added increase, (Bulk) (wt %) filter 1 (6) 50.2 20.0 2 (7) 50.8 17.7  3 (20) 46.0 19.9

Example D Paper Production

Samples of super calendered magazine quality SGW pulp as referred to above were acquired from the Kajaani paper mill in Finland. The sample was split into a coarse and a fine fraction using an OptiThick™ device in the manner described in Example A. The fine fraction had an ISO brightness of 62.4 and a yellowness of 17.6. Samples of the original SGW pulp and the coarse fraction were peroxide bleached in the manner described in Example B.

The fine fraction was co-precipitated according to standard procedures producing a 50/50 product (50 wt % original feed plus 50 wt % of newly grown carbonate) with an ISO brightness of 67.5 and a yellowness of 17.6. Due to alkali darkening of this fine fraction, over a subsequent two-week period, the original brightness fell by 5.8 units.

Hand sheets (330 mm by 330 mm) were made using an automatic sheet moulder with re-circulation following standard procedures. The target grammage was 52 g/m². One set of sheets, for the purposes of comparison, was made using the bleached original Stone GroundWood pulp. These comparison sheets were filled with either (i) sc-filler clay suitable for use in commercially available super calendered magazine paper, for example, Goonvean Platinum White or (ii) scalenohedral sc-PCC (precipitated calcium carbonate used commercially in super calendered magazine paper production).

A second set of sheets, referred to as the trial sheets, were made combining the bleached coarse fraction and the co-precipitated fine fraction so that the fibre composition matched that of the original Stone GroundWood pulp.

The hand sheets were super calendered with a Gradek 1-nip laboratory calender (hard and soft paper cylinder) under standard calendaring conditions:

Instrument: Gradek 1-nip laboratory calender;

Calendering Pressure: 40 bar;

Nips: 2+5 nips;

Temperature: 60° C.

Light scattering and light absorption results for the sheets are presented in Table 8 and, for the purposes of comparison, sheet scattering and sheet absorption results are provided for the co-precipitated fine fraction and the original fine fraction.

TABLE 8 Sheet scattering coefficient Sheet scattering Sheet absorption Sheet absorption Hand (m²/kg) for coefficient (m²/kg) coefficient coefficient sheet uncalendered for super calendered (m²/kg) for (m²/kg) for super sample sheet sheet uncalendered sheet calendered sheet Filler sc- 77.6 50.2 1.0 1.1 clay (kaolin) sc-PCC 100.3 51.0 0.8 0.9 Trial 94.1 57.5 2.8 2.8

Super calendering reduces the number of scattering surfaces by compacting the paper. This can be seen in the reduction of the scattering coefficient when paper is supercalendered. It is evident that alkaline darkening has increased the absorption coefficient of the fines containing sheet to more than double of the others, hence the low brightness of the trial paper.

Tables 9 and 10 shows the result of standard tensile and tear tests performed on the hand sheets prepared according to Example D for uncalendered and calendered hand sheets respectively. Scott Bond is a measure of the surface strength and tensile stiffness index gives an indication of the bending resistance.

TABLE 9 Hand sheet Scott sample Tensile index Tear index Tensile stiffness Bond (uncalendered) (kNm/kg) (mN/m²) index (MNm/kg) (J/m²) Filler sc-clay 22.5 4.6 2.99 206 (kaolin) sc-PCC 19.0 4.3 2.57 153 Trial 23.5 4.6 2.96 198

TABLE 10 Hand sheet Scott sample (super Tensile index Tear index Tensile stiffness Bond calendered) (kNm/kg) (mN/m²) index (MNm/kg) (J/m²) Filler sc-clay 22.6 4.0 2.86 214 (kaolin) sc-PCC 20.4 3.8 2.67 199 Trial 23.8 3.9 2.95 252

All of the measured strength properties of the trial paper were above those of so-called ‘loose’ PCC containing paper. Higher strength could allow savings in costs by reduction in chemical pulp content, which is generally the most expensive raw material in paper production, or by increasing the amount of mineral loading, which is generally the cheapest component. Increasing mineral content is generally beneficial to the brightness of the paper. It is considered that a combination of increasing the mineral content and decreasing the amount of pulp content that needs to be chemically treated, will lead to a decrease in the porosity of paper, which will be particularly beneficial in super calendered papers for rotogravure printing. 

1. A method of treating a fibre pulp mixture comprising: (a) separating the fibre pulp mixture into at least two fractions to form a coarse fraction and a fine fraction; (b) bleaching the coarse fraction from (a); (c) precipitating an alkaline earth metal carbonate in the fine fraction from (a).
 2. A method according to claim 1, wherein prior to bleaching the coarse fraction in (b) the coarse fraction is dewatered.
 3. A method according to claim 1, wherein prior to (c) the fine fraction is dewatered.
 4. A method according to claim 1, wherein the fine fraction consists predominantly of organic material that will pass through a substantially round hole of 50 μm.
 5. A method according to claim 1, wherein the fine fraction obtained in (a) is 15 to 50% by weight, preferably 20 to 30% by weight of the fibre pulp mixture.
 6. A method according to claim 1, wherein in (c) the fine solids present in the fine fraction become entrained in the alkaline earth metal carbonate precipitate to form a mixed aggregate material.
 7. A method according to claim 1, wherein the alkaline earth metal carbonate precipitate is formed by introducing into the fine fraction a source of alkaline earth metal ions and a source of carbonate ions.
 8. A method according to claim 7, wherein the source of alkaline earth metal ions is added to the fine fraction first followed by a source of carbonate ions.
 9. A method according to claim 7, wherein the source of carbonate ions is added to the fine fraction first followed by a source of alkaline earth metal ions.
 10. A method according to claim 7, wherein the source of alkaline earth metal ions is an alkaline earth metal hydroxide or a water-soluble alkaline earth metal salt.
 11. A method according to claim 10, wherein the source of alkaline earth metal ions is the alkaline earth metal hydroxide which is added ready prepared to the fine fraction.
 12. A method according to claim 10, wherein the source of alkaline earth metal ions is the alkaline earth metal hydroxide which is prepared in situ.
 13. A method according to claim 7, wherein the source of carbonate ions is carbon dioxide gas which is introduced into the fine fraction or the fine fraction containing the source of alkaline earth metal ions.
 14. A method according to claim 13 wherein the source of the carbon dioxide gas is flue gas.
 15. A method according to claim 7, wherein the source of carbonate ions is an alkali metal or ammonium carbonate.
 16. A method according to claim 1, wherein the alkaline earth metal is calcium.
 17. A method according to claim 1, wherein bleaching is carried out using an oxidising bleaching agent.
 18. A method according to claim 17, wherein the bleaching agent is selected from hydrogen peroxide, peracetic acid or ozone.
 19. A method according to claim 1, wherein bleaching is carried out using a reducing bleaching agent.
 20. A method according to claim 19, wherein the bleaching agent is selected from sulphur dioxide, dithionite, zinc dust, thiourea dioxide.
 21. A method according to claim 1, wherein the fibre pulp mixture is selected from SGW pulp, PGW pulp, TMP pulp, TRMP pulp, and DIP.
 22. Use of the mixed aggregate material obtained in step (c) according to claim 1 in a paper composition.
 23. Use of a mixed aggregate material according to claim 22, wherein the aggregated material is incorporated in the form of a suspension.
 24. A paper composition obtained from the method according to claim
 22. 25. A method according to claim 1, further comprising combining the bleached coarse fraction from (b) with some or all of the product obtained from (c).
 26. A method according to claim 25, wherein the bleached coarse fraction and the product obtained from (c) are combined in a paper making process.
 27. A method according to claim 26, wherein the bleached coarse fraction and the product obtained from (c) are blended with one or more of kaolin, clay minerals other than kaolin, talc, calcium sulphate, cellulosic fibres, a retention aid, and a sizing agent in a paper making process.
 28. A method of treating a fibre pulp mixture according to claim 1, wherein the coarse fraction from (b) is further processed in a paper making process.
 29. Paper obtained according to the method of claim
 26. 30. A method according to claim 22, further comprising coating the paper with a paper coating composition.
 31. A method according to claim 22 further comprising calendering or super calendering the paper.
 32. Calendered or super calendered paper obtained according to the method of claim
 31. 